FISHERIES ASSESSMENT TEAM
The Teifi counter was commissioned in 1998 with the
primary objective of providing data to
assess the influence of environmental variables, such as river flow, on fish
migration into the lower river. A secondary objective was to provide an
estimate of the total numbers of adult salmonids entering the river. Following
initial modelling and analysis undertaken in 2006, a number of issues and
problems with the counter were identified. These problems have prevented the
use of the counter data for stock and spawning compliance assessment. This
report summarises the work undertaken to address some of these issues during
2007 and the results obtained during the ninth year of full operational
deployment. A full review of the performance and future operation of the Teifi
counter will be produced in 2008.
Objectives for 2007
The
main objectives for 2007 were:
Validation – estimating detection rates
Table1: Proportion
of fish detected by the counter at different ranges in 2007.
|
Range (m) |
Fish observed with DIDSON |
Fish detected by counter |
Detection rate(%) |
|
0-9 |
141 |
29 |
20.6 |
|
10-13 |
82 |
44 |
53.7 |
|
13-20 |
119 |
14 |
11.8 |
Figure 1: Example
still of a shoal of sea trout recorded on DIDSON.
The proportion of fish detected by the counter during 2007
varied from 11.8% at ranges beyond 13 metres from the transducer to a maximum
of 53.7% at 10 to 13 metres from the transducer (Table 1). Detection rates were
considerably lower than in 2006 when values were generally between 22.7% and
73.1%. This reduction was due to a shift in the river profile during the winter
of 2006/7. The influence of river profile on fish detection can be clearly seen
in Figure 2, where the number of fish detected increases as the distance
between the river bed and acoustic beam decreases.
Figure 2: River depth
profile at the counter site in 2007 and the numbers of fish detected and not
detected at different ranges from the transducer (the acoustic beam or
detection zone is shown in blue).
Assessment
of fish behaviour
As split beam hydroacoustic counters are unable to
differentiate between downstream moving fish and debris they cannot produce a
downstream count. Fish holding or milling in
the area of the transducer can lead to the counter overestimating the run of
fish due to fish being counted more than once, when they move back and forth in
front of the transducer. To assess the extent of this behaviour the proportion
of fish movement in an upstream direction as observed with the DIDSON and video cameras was
estimated for different flows (Figure 3).
The results indicate that there is significant milling
behaviour at most flows, but tends to be greatest during periods of low flow.
This suggests that the extent to which the counter overestimates the run is
related to river flow. A method to estimate or directly measure the downstream
count is therefore essential for the counter to produce meaningful results.
Figure 3: Proportion
of fish moving upstream at different flows based on data collected with a
DIDSON and video between 2001 and 2007.
Deployment
of second transducer
The deployment of the second transducer was not possible
during 2007 due to safety concerns about the stanchion which suspends the
cables over the river. These cables are essential to power the equipment and
will hopefully be deployed during 2008.
Fish Counts
The counter was
operational for 83% of 2007 and the total validated count for the year was 46822
fish compared to an annual average 27079 (Table 2). The majority of fish were
counted in June, September and October with particularly low counts in July. The
disparity between the counts of 2007 and previous years is due to the unusual
flows encountered with a very wet summer and dry autumn. The counter is
known to underestimate fish migration during high flows as fish tend to move
along the right bank where they cannot be detected by the transducer. In
contrast, the low flows in the autumn would have caused an increase in
localised milling behaviour and overestimated fish counts. Both these factors are being addressed by the
deployment of a second transducer and the development of methods to estimate
downstream movement.
Table 2: Monthly breakdown of
provisional counts from the Glanteifi counter for 2006.
|
Month |
Long term validated
average (1999-2007) |
Validated count for 2007 |
|
Jan |
319 |
45 |
|
Feb |
155 |
46 |
|
Mar |
150 |
114 |
|
Apr |
1558 |
4919 |
|
May |
2266 |
4638 |
|
Jun |
5520 |
7701 |
|
Jul |
5498 |
634* |
|
Aug |
3830 |
4281 |
|
Sep |
3242 |
7280 |
|
Oct |
2677 |
12100* |
|
Nov |
1225 |
4252 |
|
Dec |
640 |
812 |
|
Total |
27079 |
46822 |
* See text for comments
Figure 4: Historical annual estimates of the total number of fish migrating past the counter.
Future Work
·
Past DIDSON deployments have identified a substantial number of fish avoiding
the counter during high flows by migrating close to the right-bank. A second
wide angled transducer will be deployed in 2008 to monitor this area
·
Due to improvements in the validation methodology the proportion of
fish detected by the counter will now be assessed on an annual basis.
·
A feasibility study will be undertaken in 2008 to assess the potential
for using the DIDSON as a long term monitoring tool at the site. It will be
assessed for use as a counter and as a tool for providing detailed information
(fish size and downstream movement) on a daily basis alongside the split beam
transducers.
·
Methods for estimating downstream movement at the site will be
assessed.
·
Work is continuing on refining a species apportionment model to split
the counts between salmon and sea trout.